In recent years, various electronic key systems such as a smart entry system are used in vehicles. In these systems, a vehicle-mounted device performs radio communication with a portable device (electronic key) carried by a user to verify ID of the portable device and control locking/unlocking of vehicle doors in response to commands from the portable device.
JP 11-71948A discloses a vehicle-side transmitter device suitable for such electronic key systems. This transmitter device variably sets a range of reach of a radio wave (searching radio wave), which is transmitted in search for a portable device. This transmitter device has an oscillator for generating a fixed output of a transmission carrier wave signal, and a signal amplifier for converting the oscillator output to a radio wave to be outputted from an antenna. For adjusting the output level of the radio wave to variably set the range of reach of the radio wave, the following two methods are proposed: (A) adjustment of the output of the signal amplifier by a variable resistor provided at an output stage of the signal amplifier; and (B) adjustment of a gain of the signal amplifier.
According to any of the methods (A) and (B), a large output type amplifier is required so that its output is directly used to drive the antenna. According to the method (A), the variable resistor causes poor power efficiency because of power loss, particularly in low power side. According to the method (B), because even a small variation in an input signal is amplified, the amplitude of the radio wave transmitted from the antenna is varied by an operation characteristic or temperature characteristic of the signal amplifier, thus resulting in unstable operation.